Gradient‐echo and spin‐echo blood oxygenation level–dependent functional MRI at ultrahigh fields of 9.4 and 15.2 Tesla

Purpose Sensitivity and specificity of blood oxygenation level–dependent (BOLD) functional MRI (fMRI) is sensitive to magnetic field strength and acquisition methods. We have investigated gradient‐echo (GE)‐ and spin‐echo (SE)‐BOLD fMRI at ultrahigh fields of 9.4 and 15.2 Tesla. Methods BOLD fMRI experiments responding to forepaw stimulation were performed with 3 echo times (TE) at each echo type and B 0 in α‐chloralose–anesthetized rats. The contralateral forelimb somatosensory region was selected for quantitative analyses. Results At 9.4 T and 15.2 T, average baseline T 2 * ( n = 9) was 26.6 and 17.1 msec, whereas baseline T 2 value ( n = 9) was 35.7 and 24.5 msec, respectively. Averaged stimulation‐induced Δ R 2 * was –1.72 s –1 at 9.4 T and –3.09 s –1 at 15.2 T, whereas Δ R 2 was –1.19 s –1 at 9.4 T and –1.97 s –1 at 15.2 T. At the optimal TE of tissue T 2 * or T 2 , BOLD percent changes were slightly higher at 15.2 T than at 9.4 T (GE: 7.4% versus 6.4% and SE: 5.7% versus 5.4%). The Δ R 2 * and Δ R 2 ratio of 15.2 T to 9.4 T was 1.8 and 1.66, respectively. The ratio of the macrovessel‐containing superficial to microvessel‐dominant parenchymal BOLD signal was 1.73 to 1.76 for GE‐BOLD versus 1.13 to 1.19 for SE‐BOLD, indicating that the SE‐BOLD contrast is less sensitive to macrovessels than GE‐BOLD. Conclusion SE‐BOLD fMRI improves spatial specificity to microvessels compared to GE‐BOLD at both fields. BOLD sensitivity is similar at the both fields and can be improved at ultrahigh fields only for thermal‐noise–dominant ultrahigh‐resolution fMRI.